A microelectronic mechanical system (MEMS) infrared (IR) sensor may have a cavity in a substrate underlapping an overlying layer. A temperature sensing component may be disposed in the overlying layer over the cavity, so that the cavity provides thermal isolation between the temperature sensing component and the substrate. It may be desirable to prevent foreign material from entering the cavity, for example through an access hole in the overlying layer, during fabrication steps of the IR sensor subsequent to forming the cavity.